Risk-based approach best for frost management
While frost is nothing new in the Australian landscape, many growers reported the frosts of 2014 were more widespread and more severe than others previously experienced. From the Mid North of South Australia through the Victorian Mallee and into southern New South Wales, abnormally cold conditions resulted in multiple frosts in early August.
But it was the record cold temperatures and the fact they lasted for many hours that surprised growers. Some crops were damaged beyond repair and cut for hay while others were left for the tillers to re-shoot. And if that damage wasn’t bad enough, there was a late frost in October which further damaged crops.
Agrilink Agricultural Consultants’ Mick Faulkner, based near Clare in the Mid North, where growers experienced huge losses to frost this year, says frost is an emotive issue – which makes its successful management all the more difficult.
“With drought, it’s a gradual thing, you know there hasn’t been much rain, and there might be none in the forecast, and the situation develops slowly. With frost, it’s sudden. A crop can go from healthy and growing well one night to dying the next morning. It’s a very challenging situation for any grower to face,” he said.
In response to the severe frosts in 2014, the GRDC held rapid response technical workshops at Clare, Kimba and Loxton in SA and West Wyalong and Finley in southern NSW, to help growers, advisers and consultants better identify and manage frost.
Frost can occur during any stage of growth, but the susceptibility of plant material depends on the growth stage. Speaking at the West Wyalong GRDC workshops, crop physiologist Neil Fettell, who is also a member of the GRDC Southern Panel, says frost damage before growth stage 30 is rare in Australia.
“Up to GS30, plants can suffer from vegetative frost, which mostly causes loss of photosynthetic area and slows growth, but in extreme cases can cause plant death. This kind of damage is uncommon in Australia because we rarely see temperatures low enough for the vegetative material to be seriously damaged,” he said.
“From GS30 (stem elongation) to GS49 (beginning of ear emergence), there can be elongation or stem frost. This is not common, but is the type that caused large amounts of damage in 2014. This can directly damage the developing ear or ‘ringbark’ the stem below the ear, resulting in ear death. The symptoms are not immediately visible without dissection.
“After GS49, during ear emergence and flowering, is the time where plants are most susceptible to frost, and this is the period growers are quite aware of as frost risk. The lesson learnt this year is that while GS59-70 is the most susceptible time, plants can suffer damage during other growth stages if the climatic conditions are severe enough,” Dr Fettell said.
Frost after flowering is also possible. During early grain fill, frost can damage the kernels resulting in shrivelled and green-coloured grain at harvest. This occurred in south-eastern NSW in 2013 and in South Australia’s Mid North in 2014, but is only occasionally a problem.
Since cold air is heavier than warm air, it sinks and flows to low areas in a similar way that water runs downslope until it ponds at the bottom. Growers should first inspect crops in low areas, where the flow of air might have ponded, for damage. Light coloured and light textured soils also produce lower temperatures in the air above them than heavier, darker soils if there is some soil water.
“Many growers used to rely on seeing frost on the ground early in the morning to indicate they had just had a frost. However the frost may have disappeared before you venture outside to see it,” Mr Faulkner said. “A more reliable method is to use paddock based weather stations or data loggers.. However, just knowing that freezing temperatures were reached does not guarantee that plant cells were frozen and damaged. Data from the Bureau of Meteorology can be somewhat useful but is measured in town locations at 1.2m above ground in a Stevenson Screen. Growers will invariably experience temperatures lower than the nearest weather station, so if the BOM data says the conditions for a frost were not quite reached, it’s still worth checking crops.”
Some symptoms will appear within the first 24 hours, such as blistering of pea pods, but most impacts will take at least a few days to appear.
“Growers should consider tagging crops after a frost event to monitor any lack of growth – plants should be growing by about 1 centimetre per day, so after two days it should be easy to spot if a plant has grown by 2cm or not. Impacts on grain should be apparent after 3-4 days, as grain should be growing at roughly 0.5 to 0.8 mm per day.” Mr Faulkner said. Another method to identify damage to vegetative parts is to remove plants, with roots intact, and place in a bucket of dyed water for a couple of days. Dye will move to all parts except those that have been damaged.
The more time that has passed since the frost event, the easier it will be to observe symptoms, but fewer management options will be available. Regular inspection of crops, particularly in low-lying areas, is recommended to identify frost damage as early as possible.
Influences on risk
Agrilink consultant Jeff Braun says after crops have been frosted, growers can improve the potential outcome by understanding the influences on frost risk.
“Cold air dams are a combination of topography and landscape. These allow a bank of cold air to accumulate which increases the risk of frost damage. Action can be taken where growers identify these dams, such as where a row of trees is blocking cold air drainage, some may be able to be removed to allow the air through, or where there is a valley between two rises, growers could choose to plant a more frost-tolerant crop or hay,” he said.
Mr Braun recommends growers take a risk-based approach, considering potential yield, frost risk and heat stress.
“The time of sowing is critical for frost and heat stress – if the crop matures too early you may get frosted, too late and it may experience heat shock. Rather than aiming for the maximum yield for every paddock every year, a balance of sowing times will provide a broader range of maturity and spread the risk,” he said.
Models such as Yield Prophet can help identify optimal times to sow varieties based on historical data, however as frost is a random occurrence, there are still no guarantees.
“Many growers think of early sowing as being negative for frost, however a later frost can mean those varieties sown earlier can escape the late event, so there’s no way to completely remove frost risk, it can only be managed,” Mr Braun said. “A thought to consider is that 10 percent loss every year due to some frost damage or compromising yields is better than 100 percent frost loss one year in ten from not diversifying.”
Mr Faulkner says where growers are sowing early, they should remember the risks.
“Work gentlemen’s hours during early sowing,” Mr Faulkner said. “If you sow your entire wheat crop on the same day, it will all be at the same stage if there is a severe frost and it’s possible to lose most of your crop.”
A GRDC-funded, five-year research project led by Melissa Rebbeck at the South Australian Research and Development Institute tested various agronomic methods for reducing frost risk. The most successful were:
Altering the soil texture through clay spreading or delving to allow the soil to store more moisture and hold more heat – reduced risk by up to 80 percent.
- Cross-sowing to improve the canopy density – reduced risk by 13 percent.
- Variety selection to promote later flowering – reduced risk by 12 percent.
- Rolling soils may have a small effect on reducing frost risk but in SARDI’s trials, no statistically significant effect was identified.
Removing stubble, wide row spacings and lower seeding rate were all trialled but were not found to reduce frost risk.
Mr Braun recommends sowing a wheat program with varieties that differ significantly from one another in the ways in which they develop.
“Currently, two commonly grown wheats in southern Australia are Mace and Cobra. Their developmental patterns are similar, which means if they’re sown around the same time they can be very close in growth stage so they will have a similar susceptibility to a severe frost event,” Mr Braun said.
There are three main maturity drivers within wheat crops:
- Thermal time – crop development depends on the cumulative temperature the plant is exposed to from sowing.
- Photoperiod – the plant requires a certain night-length and day-length balance before maturing,
- Vernalisation – plants need to be exposed a certain amount of low temperature to mature
“By growing a thermal time-driven variety and a photoperiod-driven variety, the risk of them being at similar growth stage is reduced, and so the chances of a significant loss may be reduced,” Mr Braun said.
Genetic traits for maturity in wheat can be sourced from breeding companies. Research by the University of Adelaide has shown significant variance in frost risk between wheat varieties. While the research is still underway, results-to-date demonstrate there are significant differences in susceptibility (Figure 1).
Frost in 2014
A combination of a warm May and extremely cold conditions in August caused significant frost damage in parts of south-east Australia this year. Warm conditions in May put crop maturity ahead of average for thermal-time varieties. This meant crops were more advanced than most years by the end of July, when the first frost events occurred.
“Unusual weather patterns brought a pool of very cold air from south-west of Tasmania into the grain-growing regions. This resulted in a series of frost events across August,” Mr Faulkner said.
“There were 20 or more frost events in August in many parts of south-eastern Australia. In the middle of winter, the temperature was already starting from a low point, so this ingress of cold air resulted in extremely low temperatures, which persisted for a long time. One night at Merilden in the Mid North, it was below zero for more than 15 hours with a minimum reached at about -5°C,” he said. Reports of -6 to -10°C at crop height in the Upper North have been received.
These sub-zero temperatures resulted in frost damage in plants after growth stage 30, and many areas saw 100 percent crop damage. In cases where stem frost occurred, some re-tillering occurred, however this recovery was limited by the dry finish this year.
Near the end of the growing season, a late frost occurred in the Mid North over three nights from October 14 to 16.
“As with the earlier frost, most of the plants damaged were not at flowering but at grain fill,” Mr Faulkner said. “I’ve also observed some stem frost damage from the October frost, even in plants that were flowering. This second frost serries highlights that it is just not possible to move flowering to avoid frost damage. Frost is random in nature, so the only way to manage your risk is to understand frost, and adopt risk mitigation strategies.”
Neil Fettell, 0427 201 939, email@example.com
Mick Faulkner, 0428 857 378, firstname.lastname@example.org
Jeff Braun, 0409 094 936, email@example.com
GRDC Project Code UA00136, CSP00143
Region South, North